1. Field of the Invention
The invention relates to a process for processing mixtures which contain HCl, H2O and siloxanes, in the presence of packings comprising fluorinated vinyl polymers.
2. Background Art
The preparation of silicone oils or polyorganosiloxanes is effected by polymerization of cyclic or linear polydiorganosiloxane intermediates which are produced by hydrolysis and polycondensation of diorganodichlorosilanes. Hydrochloric acid is often used as a source of water which is required for the hydrolysis. The hydrogen chloride obtained in the hydrolysis is reacted with methanol to give chloromethane and is reused in the synthesis of dimethyldichlorosilane by the direct synthesis process. Mixtures which contain HCl, H2O, and siloxanes are obtained thereby.
For example, U.S. Pat. Nos. 3,493,595 and 5,476,916 describe the preparation of α,ω-dihydroxypolydiorganosiloxanes from a crude-hydrolysis product which consists of cyclic and linear chlorine-containing polydiorganosiloxanes. For reducing the chlorine content and for removing cyclic substances, the crude hydrolysis product is treated with steam by a countercurrent method. The preparation of silicone oils or polyorganosiloxanes is expediently effected in a packed column, generally a column filled with ceramic packings. The ceramic consists of aluminosilicates with accompanying oxides of various metals such as Ti, Fe, K, Na, Ca and Mg. For example, saddle elements, Berl saddle elements, Pall rings, or special forms comprising ceramic may be used.
It has been found that the ceramic is chemically attacked under the prevailing process conditions. In particular, Al and Fe are dissolved out of the ceramic in the vicinity of the surface. This results in material defects and the formation of cavities, cracks and fissures in the aluminosilicate structure. The surface properties change; the mechanical stability of the packings decreases; and the packings easily break. A greater amount of abraded packing, which is difficult to separate, is discharged with the bottom product of the column. The separation efficiency of the column also decreases steadily. The packings have to be completely exchanged at intervals of 2 to 3 years, requiring downtime and expenses for purchase of new packings and disposal of the used packings.
The Al and Fe compounds formed, such as chlorides, oxychlorides and hydroxychlorides, remain in dissolved or suspended form in the α,ω-dihydroxypolydiorganosiloxane. These compounds act as condensation catalysts and cause undesired postcondensation of the α,ω-dihydroxypolydiorganosiloxanes. The viscosity increases in an unpredictable manner. Water which separates out makes the products cloudy and impairs their storage stability. The further processing to give a polymer is complicated by the varying quality, the varying water content and the Al and Fe compounds present.